Cameras are deceptively simple on the surface. Pick up your DSLR, look through the viewfinder, lock focus, and shoot. There’s a lot more going on behind this, including a process called Phase Detection Autofocus.
This phrase is often thrown around in the DSLR world, especially when buying a new camera. But many don’t know what this is or how it works!
What Is Autofocus?
Let’s start with the basics. There are two types of focus: auto and manual.
Manual focus is when the user has to control the focus by turning the focusing ring left or right to achieve focus.
Autofocus is when the camera does this all for you. It uses a computer to run a miniature motor that will turn the focus ring.
This focus ring moves an internal component of the lens in and out. This action repeats until the sharpest image of the subject is projected. But let’s break it down into a more detailed explanation.
All digital cameras have a histogram that tells you about what you are photographing. Contrast autofocus works by having this histogram (which communicates with the sensor) evaluated. The camera then moves the lens incrementally. It keeps reevaluating to see if there is more or less contrast to what you are shooting.
If the camera detects contrast increase, it moves the lens in that higher contrast direction until it hits its full potential. If the contrast is decreased, the camera moves the lens in the other direction.
This process is repeated over and over again until there is high contrast. Contrast autofocus exists because a well-focused image will have high contrast.
With phase detection autofocus, think a bit of the moon and its various phases. For the camera, when a specific point finds itself being in perfect focus, there are light rays.
A photograph that is in focus will have light rays that will cast a light on the opposite sides of the lens. This is when the term “in phase” comes about, like how phases of the moon work.
The camera can figure out when the focus is not achieved because the opposite side is no longer illuminated (known as not being in phase). This happens when the lens is not focused correctly on the point. It may be in front of or behind it.
How to Understand Phase Detection Autofocus
Cameras have prisms inside of them. For phase detection, the picture you see hits the prism and then separates into two images. If these images line up together, this means that your subject is in focus! If they do not, then your subject is out of focus.
So how do you get something in focus in this case? It sounds a lot like a guessing game, doesn’t it? Nope! The sensor inside the camera is aware of which split image is which. As such, it can talk to the camera and let it know which direction it should move the focus to ensure the images line up.
To make it more advanced, if you remember us mentioning light rays above- light rays pass through the lens. This light gets detected by the sensor. The system can then determine if the subject is focused on the front or the back. The camera receives direct information on how it should turn the focus ring to lock on the subject.
Fun fact: Part of the reason that DSLR cameras are so heavy is that they have an actual prism inside of them. This splits the image onto the focus sensor. Mirrorless cameras are smaller and lighter because they achieve the same result by doing this on the sensor.
Once the images are aligned, the system sends a confirmation message that the subject is in focus. All of this complexity happens in a fraction of a second!
Phase detection autofocus is great for capturing movement because it is so fast.
Now, if you’ve ever been in the buying game, you’ve likely heard about phase detection points. A digital camera has a certain number of phase detection points. There are that many sensor points where it can compare the split image. The more of these you have, the more accurate focus is.
What Is Phase Detection Autofocus Best For?
Phase detection autofocus is very well suited for action photography. It works best when used with image tracking, multiple phase detection, and AI Servo/Continuous Focus modes.
Phase detection works for other types of photography, such as portraits and still life. But action photographers will be most grateful for the inclusion of this system.
The advantages of phase-detection autofocus include being fast and allowing the sensor to assess the image’s depth of field.
You get an accurate idea of how the depth of field will look before you even take a shot.
Knowledge is never something to regret – the more you know, the larger your tool kit!
Understanding how phase detection autofocus works is useful. You can get rid of focusing issues, and know when something isn’t working correctly in your camera.